1. Field of the Invention
This invention relates to a variable resistor and particularly to a variable resistor that utilizes an extension type conductive elastomer, where the electrical resistance value is changed responsive to an applied pushing force.
2. Prior Art
Heretofore, pressure sensitive conductive rubber (PCR) has been most typically used in a device that changes its electrical resistance value responsive to a pushing force applied thereto.
Such conventional PCR, however, has the following shortcomings:
(a) Its resistance, when compressed, decreases so extremely sharp that it essentially can present only two discrete a considerably high resistance state (non-conductive state) and a considerably low resistance state (highly conductive state); and PA1 (b) The resistance value is not stable for the force applied.
Therefore, conventional apparatus that utilize PCR, have heretofore generally been used to provide an on/off function in response to whether or not a compressive force is applied thereto, rather than an analog-like type function as is reliably performed by the present invention.
Japanese Patent Publication No. Sho 60-33138, discloses a Japanese PCR that can provide an analog-like function. The PCR of this publication, however, apparently exhibits a lag or hysteresis between its electrical resistance value and the compressive force applied thereto, when such force is quickly increased or decreased. This lag of change in resistance is caused by a difference in the contact resistances of the rubber and electrodes relative to the change in compressive force. Accordingly, the apparent electrical resistance of the rubber for the same magnitude of the force will not necessarily be the same, as will be discussed below.
A U.S. patent application of one of the present inventors, entitled "Extension Type Conductive Rubber and Process for Making and Method for Using Same", filed contemporaneously herewith describes an arrangement of the PCR for use in a variable resistor or sensor.
To form such PCR variable resistor, a sheet of the PCR is sandwiched between a pair of opposing electrodes. Which electrodes are arranged such that when a force is applied to compress the rubber, the electrical resistance across the electrodes will be decreased in response to the magnitude of such force. In such a device, the electrical resistance across the electrodes involves contact resistances between the electrode surfaces and the rubber as well as the electrical resistance of the rubber itself. Accordingly, as a compressive force is increased, there will occur a slip between surfaces of the electrodes and those of the rubber, bringing the electrodes into closer contact with that rubber. This action decreases the contact resistances between the rubber and electrodes. Such slippage will also occur as the compressive force is decreased diminishing the contact between the electrode surface and the rubber, thereby increasing the contact resistance therebetween. Additionally, however, when the compressive force on the rubber surface is increased, a mechanical frictional resistance will exist between the electrodes and rubber that delays the slip that would result in a decrease in the contact resistance. This delays the decrease in the electrical resistance across the electrodes as occurs due to slippage between the surfaces. Similarly, when the compressive force is decreased, the frictional resistances will delay the slip, and the resultant increase in contact resistances. This effect delays an increase of the electrical resistance across the electrodes. The relation between the application and removal of compressive force and the electrical resistance across the electrodes thereby presents a hysteresis, and accordingly, the apparent electrical resistance of the rubber for the same magnitude of the force will not necessarily be consistent. This inconsistency is solved by the present invention.